1. Field of the Invention
The present invention relates in general to the measurement of low levels of carbon dioxide dissolved in water, and in particular, is directed to a method and apparatus for measuring low levels of carbon dioxide dissolved in water employing membrane separation and trans-membrane ion exchange techniques.
2. Description of the Related Art
In monitoring power plant water chemistry, there are three electrical conductivity measurements of interest. These are specific conductivity, cation conductivity, and degassed cation conductivity.
The electrical conductivity or conductivity, which is a measure of the ionic content of a sample, is a non-specific measurement in that all of the ions in solution, both cations and anions, contribute to the total observed conductivity. Specific conductivity is the conductivity measurement obtained from an untreated sample. When the sample is passed through a cation exchange column, which exchanges all cations in solution for the most highly conductive cation, i.e., the hydrogen ion, the cation conductivity is then measured. After the sample passes through the cation exchange column, all of the anions are in their acid form. The degassed cation conductivity measurement is normally the third in this series of three conductivity measurements.
The degassed cation conductivity is a measure of the cation conductivity of a sample after all of the volatile substances which contribute to the conductivity are removed. Normally, the solution is heated to near the boiling point of water to expel carbon dioxide and the other volatile components. The conductivity of the sample is measured at the high temperature and one of several algorithms is used to relate the high temperature conductivity to the room temperature conductivity. Alternatively, the degassed sample can be cooled to room temperature before measuring the conductivity.
The current method for measuring low levels of carbon dioxide dissolved in water is the degassed cation conductivity method which is also known as the reboiler method. This method assumes that the main volatile constituent contributing to conductivity is the dissolved carbon dioxide which exists in solution as carbonic acid. In this way, the degassed cation conductivity when compared to cation conductivity is an indication of the carbon dioxide content of the sample. In power plants, these measurements allow for distinguishing conductivity increases due to cooling water leaks and air leakage.
There are several disadvantages to the degassed cation conductivity method. The first is that the method does not selectively remove carbon dioxide. At the high temperature, other volatile species are also driven off as well as the carbon dioxide. These include anions of interest like chloride and sulfate which have been converted to the more volatile hydrochloric acid and sulfuric acid by the cation exchange. A key premise of the degassed cation conductivity method is that these other anions are unaffected by the heating process.
Another disadvantage is that the method is probably incomplete. An air saturated water sample contains approximately 0.5 to 1 ppm of total carbonate depending on temperature and pressure from the carbon dioxide in the air. In solution, carbon dioxide is distributed among several forms including carbonate (CO.sub.3), bicarbonate (HCO.sub.3), carbonic acid (H.sub.2 CO.sub.3), and carbon dioxide in various forms of hydration. The volatile forms are the non-ionic species. At a pH of 5.7, which is a typical value for air saturated water, the non-volatile carbonate and bicarbonate constitute about 20% of the total carbonate content. Thus, at any one time, only 80% is available for removal due to volatility. Under static conditions, as carbon dioxide is removed, the 20% non-volatile portion is shifted toward the volatile forms and 100% removal is theoretically achievable. However, this method is a flowing system. Slowing the flow to aid in efficiency of carbon dioxide removal also increases the opportunity to boil off other anions leading to an erroneous result.
Still another disadvantage is that useful results require the determination of small differences in conductivity. For example, in a solution which is a 300 ppb chloride (as HCl) and 500 ppb carbonate, the theoretical conductivity is 3.764 microsiemens per centimeter (.mu.S/cm). If all the carbonate is removed, the conductivity due to 300 ppb chloride is 3.607 .mu.S/cm). Thus, the carbon dioxide content of the sample is measurable by a difference of only 0.157 .mu.S/cm.
A further disadvantage of the degassed cation conductivity method is that a practical interpretation of the conductivity measured at the outlet relies on a mathematical relationship between the conductivity at 98.degree. C. and the corresponding room temperature conductivity. The alternative method of cooling the sample back to room temperature before measurement introduces the possibility of readmitting carbon dioxide to the sample through air ingress. Additionally, the high temperature process may result in corrosion, the conductive products of which will give erroneously high conductivity measurements.
Thus there is a need for an apparatus and method which measures low levels of carbon dioxide dissolved in water without the disadvantages of the degassed cation conductivity method. A method is needed where conductivity measurements are taken at an ambient temperature to eliminate reliance on mathematical algorithms for conversion of data and which eliminates concern from potentially interfering high temperature reactions. There is a need for higher precision in the interpretation of measured data to provide confidence and useful results in measuring carbon dioxide content of the sample by increasing the conductivity measurements.
While it is known in ion chromatography how to suppress high background conductivity with membrane based cation exchange systems for greater sensitivity, heretofore there are no methods available with trans-membrane ion exchange techniques that allow for the measurement of low levels of carbon dioxide in water.